The present invention relates to a drive circuit and a drive device both suitable for use in a liquid crystal display or the like. The present invention relates particularly to a source driver of a liquid crystal display, having a large number of output op amps.
With upsizing of a recent liquid crystal display, there has been a demand for improvements in various performance of the liquid crystal display. In particular, the load capacity of a data line also increases with the upsizing of the liquid crystal display, and an improvement in the drive capacity of the liquid crystal display has been expected. Competition has heated up in recent years in the market of the liquid crystal display, and hence respective mounting parts have been forced to cut cost down. A drive device is also similar to the above. There has thus been a demand for one that has high drive capacity and is low in cost.
In a patent document 1 (Japanese Unexamined Patent Publication No. Hei 5(1993)-041651), a comparator makes a comparison of difference between input and output signals of an output op amp. Only when the input signal is lower greater than or equal to a predetermined threshold voltage than the output signal, an enable signal is outputted from the comparator to turn on a switching transistor thereby to make a large current source effective, thus making it possible to vary an output current on the discharge side of a load capacitance, whereby power consumption of a device is suppressed. In a patent document 2 (Japanese Unexamined Patent Publication No. 2003-122325), a plurality of charge/discharge paths are provided to improve a through rate.
In the above patent document 1, however, the operation is no different from the conventional output until a constant threshold value is reached, although the discharge can be done quickly when the input signal is low greater than or equal to the threshold voltage. When the range of an output voltage is wide, effectiveness becomes very low. Further, the charging operation is not set up speedily. Although the above patent document 2 has a charging operation and a discharging operation, there is a need to control their separate paths by some separate signals, thus resulting in an increase in control circuit. Even though a transistor is simply turned on/off to perform control, a transistor appropriate to a large current cannot be expected to have a quick response, thereby causing difficulty in control.